The present invention relates to manufacturing a heterobipolar transistor and a laser diode on or from the same substrate and it also relates to substrates suitable for such manufacturing. The present invention is also disclosed in the article U. Eriksson, P. Evaldsson, B. Stxc3xa5lnacke, B. Willxc3xa9n, xe2x80x9c1.55 xcexcm multiple quantum will laser and heterojunction bipolar transistor fabricated from the same structure utilizing zinc diffusionxe2x80x9d, SPIE Vol. 3006, pp. 145-152, 1997, which is included by reference herein.
The research in the field of monolithic (i.e. arranged on or in the same chip or circuit plate) optoelectronic integrated circuits (OEICs) started in the end of the seventies at CALTECH, USA, see the article by C. P. Lee, S. Margalit, I. Ury and A. Yariv, xe2x80x9cIntegration of an injection laser with a Gunn oscillator on a semi-insulating GaAs substratexe2x80x9d, Appl. Phys. Lett., Vol. 32, No. 12., pp. 806-807, June 1978. The reason thereof was the same as in developing electrical integrated silicon circuits, i.e. it is desired to manufacture both optical components such as lasers, waveguides, detectors on the same substrate as transistors, so that it could be possible to produce chips in large volumes and at low costs. Monolithic integration also reduces the number of chips what allows that more functionality can be packed into a circuit board, on which different chips are conventionally mounted. It can also increase the reliability of a system since fewer external connections are required. It should be added here that a condition for achieving these advantages is that the performance of the various components is not degraded when being integrated, compared to the case where they are manufactured separately. The interest of finding a good solution to the problem how to combine optical and electronic components on the same chip nowadays emanates not only from the technical side but also from the system side. In order to be able to build the optical networks of tomorrow, e.g. extending even into homes, less costly solutions are required, what in turn poses large demands on the technical development.
A large number of different alternative ways exist of achieving monolithic integration. It depends partly on the choice of wavelength and thereby the semiconductor base material which is to be used, such as whether to select either gallium arsenide or indium phosphide, and the choice of electric components such as whether a heterobipolar transistor (HBT) or a field effect transistor (FET) is to be used, and further the choice of optical component, which is desired, such as a photodetector, laser or modulator, partly also on the way in which the very integration is made. It is common to divide the methods used for integration in three classes:
1. Vertical integration. Two or more structures each including an electrical or optoelectrical component are formed sequentially on top of each other.
2. Horizontal integration. Two or more structures each including a component are formed side by side. First the different layers are formed for producing a component and then these are etched away on selected portions of the chip at the side of the manufactured component. The next component is then grown on areas at which material has been etched away.
3. Using the same basic structure for the two components. A basic layer structure is formed, which by further processing including for example etching for defining individual components and only including applying layers for electrical contacts but no other layers results in components of various kinds isolated from each other.
The methods 1. and 2. have the advantage that in principle the individual manufactured components can be optimized. The disadvantage is that the methods of manufacture often will be very complex including a very large number of processing steps. The method 3. results in a simpler manufacturing process but also, most often a compromise must be made as to the performance of the different components to be manufactured.
A method which has often been mentioned in the literature is, to pass, for the laser, from vertical injection to lateral injection and such a laser is called an LCI-laser (xe2x80x9cLateral Current Injection Laserxe2x80x9d). Then the different n- and p-doped layers are defined by means of diffusion or implantation, which is made selectively on different portions of a substrate surface, and thereby both lasers and transistors can be produced from the same substrate or chip. This method has been used for integrating a laser and a FET, see the above cited article by C. P. Lee et al., and of a laser and HBT, see N. Bar-Chaim, Ch. Harder, J. Katz, S. Margalit, A. Yariv, I. Ury, xe2x80x9cMonolithic integration of a GaAlAs buried-heterostructure laser and a bipolar phototransistorxe2x80x9d, Appl. Phys. Lett., 40(7), 556, (1982). A disadvantage of this method is however that the result is a new type of laser and/or transistor. Another concept, which has been used, see T. Fukuzawa, M. Nakamura, M. Hirao, T. Kuroda, and J. Umeda, xe2x80x9cMonolithic integration of a GaAlAs injection laser with a Schottky-gate field effect transistorxe2x80x9d, Appl. Phys. Lett., 36(3), 181, (1980), is to first grow the laser structure and on top thereof an undoped layer, from which a FET can be produced. In order to obtain electrical contact with the top p-layer of the laser one then diffuses a p-doping (in this case zinc) through the undoped layer.
It is an object of the invention to provide a method, by means of which it is possible to manufacture in a simple way transistors and lasers on the same substrate or chip without degrading the performance of transistors and lasers owing to the common method of manufacture. Thus, the problem solved by the invention is how to manufacture transistors and lasers on the same substrate or chip, so that the performance of the transistors and lasers thus manufactured is substantially equal to that of separately fabricated corresponding components.
When manufacturing transistors and lasers on the same substrate a basic structure is first produced, which has a suitably selected sequence of semiconducting layers arranged on top of each other and in particular the start is a substantially xe2x80x9cconventionalxe2x80x9d HBT-structure. The basic structure is then converted to a laser on some areas of the chip. The laser will thereby be the type vertical injection and will thereby be capable of obtaining the same performance as discrete lasers. The conversion to a laser structure is made by diffusing zinc into the material. The advantage therein is that one obtains substantially the same structure of the laser and HBT, as if they had been individually optimized. Similar structures have been made in GaAs/GaAlAs, see J. Katz, N. Bar-Chaim, P. C. Chen, S. Margalit, I. Ury, D. Wilt, M. Yust, A. Yariv, xe2x80x9cA monolithic integration of GaAs/GaAlAs bipolar transistor and heterostructure laserxe2x80x9d, Appl. Phys. Lett., 37(2), 211, 1980. The method proposed in that paper includes that the active area of the laser is located in the base of the HBT what is a difference compared to the method as proposed herein. In the article A. K. Goyal, M. S. Miller, S. I. Long and D. Leonard, xe2x80x9cA single epitaxial structure for the integration of lasers with HBTsxe2x80x9d, SPIE, Vol. 2148, pp. 359-366, 1994, also monolithic integration in the system GaAs/GaAlAs is used but the active region is instead located in the collector in the same way as in the design described herein, what gives a larger freedom when designing the components and what allows an individual optimization of the two components to be made.
A heterobipolar transistor HBT and a laser diode LD are manufactured from a common epitaxial structure. The transistor is then made directly from this epitaxial structure by only confining, separating, isolating and/or defining it by etching and applying electrical contact layers. The different active layers of the transistor are thus the epitaxial layers formed in the structure. In order to manufacture the laser diode the structure is changed by diffusing zinc into it, so that the topmost material layers change their types of doping from n-type to p-type. This is made in selected areas of a wafer, so that transistors and laser diodes in that way are monolithically integrated. Generally, the opposite change, i.e. from p-doping to n-doping in the upper layers, could also be used.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.